As large-scale integration (LSI) progresses toward a higher integration density and faster processing speed, reduction of pattern rules progresses rapidly. In the background of this rapid progress of downscaling, projection lenses having a higher numerical apertures (NA) are being developed, resist-composition performance is being improved, and light sources are being shifted to shorter wavelengths.
Use of the resist composition for a krypton-fluoride (KrF) excimer laser (248 nm) began in a 0.3-μm process, and this has been applied to mass-production of a 0.13-μm rule. The shift to an argon-fluoride (ArF) excimer laser (193 nm), which has a shorter wavelength than a KrF excimer laser, makes it possible to reduce the design rule to 0.13 μm or less. However resins such as a novolak resin or a polyvinyl phenol resin, which have been used in the past, have strong absorption near 193 nm, and therefore they cannot be used as a resist base resin. To secure transparency and sufficient dry-etching resistance, an acryl resin and a cycloolefin-based alicyclic resin were evaluated, and as a result, mass-production of a device using ArF lithography was realized.
In the 45-nm-node devices, the wavelength of the exposure-light source was shortened, and thus F2 lithography of 157 nm became a candidate for next-generation lithography. However, F2 lithography has several problems, such as: increased cost of scanners due to the introduction of expensive CaF2 single crystals in a projection lens; the need to change the optical system due to the extremely poor durability of soft pellicles, thereby necessitating the introduction of a hard pellicle; and decreased etching resistance of the resist. Thus, postponement of F2 lithography and early introduction of ArF immersion lithography were proposed, and 45-nm node devices using ArF immersion lithography are being produced on a mass scale. For mass-production of a 32-nm node device, a double patterning process using a side-wall spacer has been adopted, but the process is long and complicated, which is a problem.
Extreme ultraviolet (EUV) lithography of 13.5-nm wavelength, instead of an expensive double-patterning process, is expected for devices after 32 nm, the resolution thereof being improved by shifting the exposure light to a wavelength that is shorter by more than one digit than previous lithography technology, and thus the development of EUV lithography of 13.5 nm is progressing. In EUV lithography, the power of a laser is weak, and the amount of light decreases according to the attenuation of the reflective mirror light, and thus the intensity of light reaching a wafer surface is low. Thus, to acquire throughput with low light intensity, development of a highly sensitive resist is urgently needed. However, if the sensitivity of the resist is enhanced, there is a problem of deterioration in resolution and edge roughness (LER: line-edge roughness and LWR: line-width roughness), and thus a trade-off relationship between increased sensitivity and deterioration in resolution and edge roughness has been pointed out.
An EUV resist is easily affected by contaminants in the atmosphere around it because of the resist's high sensitivity. An amine quencher usually is added to a chemically amplified resist to ease the effect of amine contamination in air, but the amount of the amine quencher added to an EUV resist is a small fraction of the amount of the amine quencher added to an ArF resist. Accordingly, an EUV resist tends to form a T-top shape by receiving the effect of the amine from the resist's surface. The formation of a top coat on a resist's upperlayer is effective in preventing amine contamination. In a chemically amplified resist of an early type for a KrF excimer laser based on a t-BOC (tertiary-butoxycarbonyl)-protected polyhydroxy styrene, to which an amine quencher was not added, the use of a top coat was effective. Even in an early stage of ArF immersion lithography, a top coat was used to prevent elution of an acid generator into water, thereby preventing formation of a T-top shape that would result from such elution. Here, also in the EUV lithography process, to form a top coat on the upperlayer of a resist was proposed as described in Non-patent Document 1. By forming a top coat, environmental resistance is improved and the outgas from resist film is reduced.
An EUV laser of DPP (discharge-produced plasma) and LPP (laser-produced plasma) emits, in addition to 13.5-nm-wavelength light, which is used for patterning, a weak broadband light having a wavelength of 140 nm to 300 nm (out-of-band, OOB). The intensity of this broadband light (hereinafter “OOB light”) is weak, but the amount of the light's energy cannot be neglected because of its wide wavelength range. A Zr (zirconium) filter is provided to an EUV microstepper to cut the OOB light, but this decreases the light intensity. In an EUV scanner in which, to improve throughput, the light intensity is not allowed to decrease, it is possible to not use this filter. Because the OOB light decreases the pattern contrast that is formed by EUV light, there is a need to develop a resist material that has high sensitivity to EUV light and has low sensitivity to OOB light. In this regard, Non-patent Document 1 describes providing a top coat to absorb the OOB light on the upper layer of a resist and shows the advantage of the top coat shielding the upperlayer of the resist from the OOB light.
Many kinds of resist top coat compositions used in ArF immersion lithography have been proposed. Among them, Patent Document 1, mentioned below, discloses a top coat composition containing a repeating unit of a styrene having a 1,1,1,3,3,3-hexafluoro-2-propanol (HFA) group, which is supposed to be unpractical because it absorbs a very high amount of 193 nm-wavelength light.
It has been pointed out that, in the case of a top coat for immersion lithography, a solvent for a top coat dissolves the surface of the resist film, thereby causing mixing between the top coat and the resist film, and this in turn causes film loss of a resist pattern after the film has been developed (Patent Document 2). Especially, when an alcohol solvent is used, a great amount of film loss occurs. It has also been shown that an ether solvent or a C7-C11 hydrocarbon-based solvent, in addition to an alcohol solvent, is effective in inhibiting film loss (Patent Document 3). An example of a polymer that is soluble in ether solvents is a polymer that has a 1,1,1,3,3,3-hexafluoro-2-propanol (HFA) group, as described in Patent Document 2.
Non-patent Document 1 reports that (1) in the case of a positive-type resist film, when an entire wafer is exposed, the width of the lines in the peripheral region of a shot becomes narrower due to OOB light leaked from neighboring shots; and that (2) if a top coat that absorbs light having a wavelength of 200 nm to 300 nm is applied on a resist film, the variation of pattern size within a shot can be reduced. Patent Document 4 describes a top coat of a hydroxyl-styrene or a cresol-novolak resin applied as a solution of alcoholic solvents such as 4-methyl-2-pentanol, 2-methyl-2-pentanol, isopropyl alcohol, 3,3,5-trimethyl-1-hexanol, or a mixed solvent of one or more of the aforementioned alcoholic solvents and one or more hydrocarbon-based solvents such as xylene.